Many processes and devices require precise control over the flow volume of the fluid constituents utilized therein. The operating efficiency of such processes and devices often depends upon the accuracy and dependability of the control over the flow volume. Faulty valve calibration or positioning inaccuracies in the valve actuator caused by wear and tear may result in operational failures, irregular performance, and wasteful energy consumption.
Valve types typically used as flow control valves include cylinoid, piston, gate, globe and diaphragm. Of these types, diaphragm valves with hermetic seals to the flowing fluid have proven to be the preferred choice for applications where leakage cannot be tolerated, because these valves do not experience the leakage problems associated with the other valve types. In operation, diaphragm valves are known to be fairly reliable though their efficiency and accuracy is largely dependent upon the precision and frequency of the valve's calibration. Presently, diaphragm valves are calibrated manually through a repetitive series of minute adjustments, taking up to several hours to complete. Once calibrated, most diaphragm valves are controlled by pneumatic, i.e. air pressure, or electrical means.
Historically, fluid flow control valves were actuated by pneumatic controllers. Such controllers were desirable because they were compatible with the building control systems of that period which also operated with air pressure. Not only were the systems compatible, but the technology was well known by the maintenance personnel. However, with the coming of the computer age, building controls have converted from pneumatic systems to computer systems, and the building controls no longer have available the air to drive pneumatically controlled valves. Additionally, air pressure sensors are generally not as accurate or as precise in signal sensing as are electrical sensors, and consequently are less desirable.
Pneumatically controlled diaphragm valves are typically actuated by a continuously variable signal pressure, for instance, a pressure between 3-15 PSIG (pounds per square inch gauge). The actuator associated with a pneumatically controlled diaphragm valve usually has a mechanical stop attached to the operating stem for defining the minimum closed position. However, this method of defining the minimum closed position does not take into consideration diaphragm thickness, hardness, or machining variances in manufacturing. In operation, pneumatically controlled diaphragm valves have proven to be inaccurate and difficult to control because of hysteresis. Hysteresis is a condition where the diaphragm valve experiences a dead band for a period of time in which the diaphragm does not move subsequent to a change in control pressure.
Simple electric drive motors have also proven to be inaccurate because the amount of movement in a measured voltage pulse is a function of the resisting force in the valve which may vary depending upon whether the valve is working against easy start-up conditions or against full load pressure differentials. As a result, electric drive actuators tend to wander since they do not have means for defining the true valve actuator position.
A more precise and predictable diaphragm valve control means is the electric drive stepper motor. Stepper motors, once integrated into the appropriate linear actuator, are able to move the valve actuator linearly in increments on the order of about 0.00005 inches per motor step, providing precise control over the valve actuator position. However, a problem with the stepper motor vane actuator is determining a full closed position in steps which would not cause damage to the rubber diaphragm when the actuator is commanded to drive the vane closed. With conventional stepper motor valves, the stepper motor drives the valve closed at a given step rate until the valve forces the rubber diaphragm into the far wall of the vane body, and thereby compressing the rubber diaphragm. Once the motor is opposed by a mechanical torque greater than the motor torque and the valve actuator can move no further, the stepper motor stalls. If the stepper motor continues to step off steps, the motor will begin to rotate in the reverse direction until the torque loads are balanced. This anomaly is referred to as "fall back." Consequently, the valve is incapable of determining the actual number of steps to the full closed position and may cause irreparable harm to the rubber diaphragm in attempting to do so.
Calibrating or adjusting the full closed position of either the pneumatically controlled or electrically-controlled diaphragm valve typically requires a repetitive process comprising manual adjustments of the valve to the closed position over a plurality of iterations, while monitoring the other operating parameters until the actual full closed position is properly determined. This process may take in excess of several hours. Calibrating or adjusting the full closed position is important not only to maintain precise predictable control over solution flow during minimum open periods, but also to prevent "compression set" to a diaphragm caused by long term over compression of the valve's diaphragm which results in an inaccurate calibration or loss of calibration. Presently, there is no diaphragm valve control means which can automatically define the actual full closed position of a valve with precision in a short period of time.
The critical nature of valve calibration is evident in the operation of an absorption water chiller. Absorption water chillers, such as the Single Stage Absorption Cold Generator.RTM. manufactured by and commercially available from the Trane Company, Wisconsin, U.S.A., circulate a solution of lithium bromide (LiBr) and water. By modulating the flow volume of the LiBr solution in proportion to the load on the absorption unit during low load conditions, less energy is consumed by the absorption unit because less solution reaches the generator, and therefore, less energy is required to heat the solution. For example, the energy consumption of Trane's Single State Absorption Cold Generator.RTM. operating at 50% full load is reduced by 7% when the flow volume of the operating solution is proportionally reduced by a diaphragm vane. To achieve this degree of efficiency, the flow control valve must be highly accurate. During unit operation times the absorption unit may operate at as little as 10% full loading, requiring the diaphragm valve actuator to close to a minimum open stop of 1/16" or 1/8", depending upon the unit model. Consequently, a 1/16" error in determining the minimum open stop or the true position of the valve actuator could change the actual flow volume as much as 50% from the desired flow volume. Such inaccuracies in the valve actuator position translate into inaccuracies in the solution flow volume which result in wasteful energy consumption and may potentially result in damage to the absorption unit.
A known, defined minimum open stop valve position is required in absorption water chillers to insure some minimum amount of flow while the absorption water chiller is operating during periods of light loading. This minimum flow prevents the crystallization of the LiBr solution in the absorption water chiller, and thus prevents the solution pump from working against a blocked flow condition. Crystallization occurs when the LiBr solution solidifies, causing the flow to cease. Running the solution pump during blocked flow conditions may result in pump damage and/or nuisance motor safety trips. Consequently, there presently exists a heretofore unaddressed need in the industry for a method to more accurately calibrate diaphragm valves.